1. Field of the Invention
The present invention generally relates to multi-valve double-over-head-camshaft engines (referred to as "DOHC engines"), and more particularly to DOHC engines which have relatively large space for installation of a functional part (e.g., injector) along the center line of each of cylinder bores.
2. Description of the Related Art
One of known DOHC engines is a four-valve DOHC engine which has two intake valves and two exhaust valves for each of cylinders and which is equipped with a functional part such as injector or spark plug arranged along the center line of each of the cylinder bores. In this type of DOHC engine, when viewed from the top, intake valves are arranged generally linearly in parallel to the longitudinal center line of the engine, and the exhaust valves are also arranged linearly in parallel to the longitudinal center line of the engine on the opposite side.
In a conventional valve drive mechanism for such a four-valve DOHC engine, when viewed from the top, the center line of a camshaft for the intake valves extends right above the mutual center line of the linearly arranged intake valves, and likewise the center line of the other camshaft for the exhaust valves lies right above the mutual center line of the linearly arranged exhaust valves. Further, the center line of each cam (or cam lobe) lies right above the center of associated valve.
If an engine has relatively small bores, the distance between the intake camshaft and exhaust camshaft is relatively small, and the distance between each two cams (cam lobes) on each camshaft for each cylinder is relatively small. Therefore, the space for a functional part such as injector or spark plug to be installed along the center line of each cylinder bore is also relatively small. Similarly, if the included angle (i.e., an angle between the intake valve and exhaust valve) is small, the spacing between the intake and exhaust camshafts is small so that the installation space for the functional part is small.
An engine having five valves for each cylinder is disclosed in, for example, EP 0 444 562 A1, entitled "Timing System, Particularly For An Internal Combustion Engine With A Number Of Valves Per Cylinder" published on Sep. 4, 1991.